Recently, a reciprocating compressor has been developed to compress a refrigerant gas in a refrigerator and so on.
U.S. Pat. No. 5,342,176 discloses a reciprocating compressor using a linear motion motor and a method for controlling a piston stroke of the reciprocating compressor.
FIG. 1 is a cross sectional view illustrating the construction of a reciprocating compressor disclosed in U.S. Pat. No. 5,342,176, and FIG. 2 is a block diagram illustrating a compressor controlling apparatus for controlling a piston stroke of the refrigerant shown in FIG. 1.
According to the conventional reciprocating compressor, as shown in FIG. 1, a piston 1 reciprocates in a cylinder 2 in response to forces on magnets 4 to which the piston is connected by a yoke 3. The forces on the magnets are caused by magnetic fields set up by current in a winding 5. Piston motion is transmitted by the yoke linking the piston 1 to a spring 6, which has a spring constant K. During downward piston motion, gas or vapor at suction pressure, which is the pressure in a surrounding space 9 and also in the lower part of a compressor interior space 10, is drawn into the cylinder through a check valve 7. During upward motion of the piston, gas or vapor is initially compressed until the pressure in the cylinder exceeds the discharge pressure, that is, the pressure in a discharge pipe 11, at which a point check valve 8 opens and gas or vapor is pushed into the discharge pipe by continuing upward motion of the piston.
A conventional apparatus for controlling the reciprocating compressor as described above will now be described.
The reciprocating compressor comprises, as shown in FIG. 2, a voltage detecting section 13, connected to input terminals of winding 5, for detecting the voltage applied to the winding as a function of time, a current detecting section 12, connected to the winding 5, for detecting the current through the winding as a function of time, a computing section 14 for calculating a velocity of the piston using the voltage and current values detected by the voltage and current detecting sections 13 and 12 and operating the piston stroke from the velocity of the piston, and a commending section 15 for comparing the stroke value operated at the computing section 14 and a predetermined voltage value, determining a target output voltage to compensate the difference between the stroke value and the predetermined voltage value, and commanding it to a driving section 16.
A conventional method for controlling the prior reciprocating compressor will now be described.
Predetermined end displacement values (top and bottom dead points) are inputted.
By supplying a power to the motor of the compressor at a certain value, the voltage and current supplied to the winding of the compressor are detected as a function of time, respectively.
A displacement value of the piston is measured using the detected voltage and current.
By comparing the measured displacement value with the predetermined displacement value, an error signal corresponding to the comparison is outputted.
The voltage to be supplied to the winding of the motor is varied in corresponding to the error signal to minimize the error signal.
The step for outputting the error signal will be described.v=(1/α)(V−L(dI/dt)−IR)  [Equation 1]
wherein, α is a transfer constant, V is the voltage applied to the winding, I is the current detected from the winding, R is a winding resistance, L is a winding inductance, and t is time.
The velocity v of the reciprocating piston is calculated as a function of time from the detected voltage and current in accordance with the equation 1. The computed velocity is integrated as a function of time to compute the alternating component of displacement of the piston as a function of time. The computed velocity is differenced as a function of time to compute the acceleration of the piston as a function of time.
The alternating component of displacement is detected when the computed velocity is zero. Simultaneously, during a suction phase (moving towards the bottom dead point), the alternating component of displacement, the acceleration and the current are detected. The displacement of the reciprocating piston is calculated at the end of its excursion in accordance with a following equation 2.Xc=xi−xo+(αa/K)Io−(M/K)Ao  [Equation 2]
wherein, Xc is the end displacement, xi is the alternating displacement when the velocity is zero, xo is the simultaneously detected alternating displacement, Ao is the simultaneously detected acceleration, Io is the simultaneously detected current, M is the mass of the reciprocating body, and K is the spring constant of the spring.
By comparing the command signal with the computed end displacement signal Xc, an error signal is generated.
The prior apparatus and method for controlling the reciprocating compressor using the above displacement-voltage feedback has following disadvantages.
Firstly, because the critical value of the dead point of the displacement of the piston has to be exactly calculated, the complicated calculation of the dead point of the displacement causes to an error. Specifically, it is necessary to carry out the complicated calculation such as equations 1 and 2, thereby producing an error of the calculation.
Secondly, since expensive apparatuses such as a computer are used to carry out the complicated calculation, the cost increases.
Finally, according to the U.S. patent, after the ideal dead point of the displacement to be controlled is predetermined, the voltage is controlled in such a way that it is approached to the predetermined displacement. If the compressor is continuously used, the compressor is controlled using the predetermined displacement, in spite of the variation of the ideal displacement due to the mechanical wear. Therefore, it is impossible to exactly control the compressor.
Japanese Patent Laid-Open Publication hei 9-112438 discloses an apparatus and method for controlling the reciprocating compressor, in which an operating frequency is adjusted according to a resonance frequency so that the efficiency thereof is not reduced regardless of that a resonance frequency may be changed by the change of a spring constant of gas due to the fluctuation of the load.
FIG. 3 is a block diagram of one conventional control apparatus for the reciprocating compressor disclosed in the Japanese Patent Laid-Open Publication hei 9-112438, and FIG. 4 is a block diagram of another conventional control apparatus for a reciprocating compressor disclosed in the Japanese Patent Laid-Open Publication hei 9-112438.
The conventional control apparatus for a reciprocating compressor shown in FIG. 3 comprises an alternating power supply section 21 for supplying a driving power to the compressor 27 and having a controllable frequency of the output voltage, a voltage detecting section 22 for detecting an output voltage outputted from the alternating power supply section 21 to the compressor 27, a current detecting section 23 for detecting a current flowing>from the alternating power supply section 21 to the compressor 27, a phase detecting section 24 for detecting a phase difference between the output voltage detected from the voltage detecting section 22 and the current detected from the current detecting section 23, and a control section 25 for compensating a frequency of the output voltage of the alternating power supply section 21 corresponding to the phase difference detected from the phase detecting section 24 and coinciding the frequency with a resonance frequency of a piston of the compressor.
The conventional control method of the reciprocating compressor will now be described.
If the driving power is supplied to the reciprocating compressor 27 from the alternating power supply section 21, the reciprocating compressor 27 is driven. At that time, the voltage detecting section 22 and the current detecting section 23 detect the current and voltage applied to the compressor, respectively.
The phase detecting section 24 calculates a timing based on a waveform phase of the detected voltage value V and current value I, and calculates the phase difference Dp of the current I to the voltage V based on the calculated results.
The control section 25 calculates a frequency compensating amount ΔF corresponding to the phase difference Dp, and outputs a frequency control signal to the alternating power supply section 21 corresponding to a frequency control amount Ff (Ff=Ff+ΔF).
Even if the resonance frequency Fc of the piston is fluctuated due to the fluctuation of the load, the frequency F of the output voltage V of the alternating power supply section is controlled to be coincided to the resonance frequency Fc.
In addition, the control apparatus for a reciprocating compressor shown in FIG. 4 comprises an alternating power supply section 21 for supplying a driving power to the compressor 27 and having a controllable frequency of the output voltage, a voltage detecting section 22 for detecting an output voltage outputted from the alternating power supply section 21 to the compressor 27, a current detecting section 23 for detecting a current flowing from the alternating power supply section 21 to the compressor 27, a velocity detecting section 26 for detecting a piston velocity of the compressor 27 according to the detected results of the voltage detecting section 22 and the current detecting section 23, and a frequency control section 28 for detecting a phase difference between the current detected from the current detecting section 23 and the velocity detected from the velocity detecting section 26 to compensate the frequency of the output voltage of the alternating power supply section 21 corresponding to the detected phase difference, and coinciding the frequency with a resonance frequency of a piston of the compressor. The alternating power supply section 21 includes a DC power supply section 21a for supplying a DC power, and an inverter 21b for adjusting the frequency of the voltage outputted from the DC power supply section 21 a according to the control signal of the frequency control section 28.
The conventional control method of the reciprocating compressor will now be described.
The phase difference Dpie of the current I flowing from the alternating power supply section 21 to the compressor and the phase difference Dpve of the velocity of the piston to the voltage V are to be coincided with the resonance frequency Fc, thereby becoming zero degree. Also, if the driving frequency F is higher than the resonance frequency Fc, the phase of the current I goes ahead of that of the velocity v. If the driving frequency F is lower than the resonance frequency Fc, the phase of the current I is behind that of the velocity v. Accordingly, the compressor is controlled using the resonance frequency Fc variable depending upon the load, so that if the phase of the current I goes ahead of that of the velocity v, the driving frequency F is lowered, while if the phase of the current I goes ahead of that of the velocity v, the driving frequency F is raised.
However, the prior apparatus and method for controlling the reciprocating compressor disclosed in the Japanese Patent Laid-Open Publication has following disadvantage.
In order to control the frequency of the power supplied to the compressor, an expensive apparatus (inverter) has to be provided. Accordingly, since the cost of components is increased, it is impossible to provide an inexpensive control apparatus.